Heh. Well, I guess there's only one way to find out for sure... But QBP says:

"Due to varying dimensional tolerances between manufacturers, FSA MegaExo bottom brackets are not compatible with any other brand of cranksetDifferent models of MegaExo bottom brackets are designed to be specific to each model of FSA crankset"

Last summer I helped my friend install an Ultegra BB with a FSA crank. The fit was "very" tight. We got it in with the use of a mallet (at my friend's urging, I cringed while doing it) and it has worked fine but the fit is less than ideal. I think it is hit or miss really, sometimes they fit fine, sometimes they need help.

Last summer I helped my friend install an Ultegra BB with a FSA crank. The fit was "very" tight.

Doesn't sound like a BB compatibility issue but rather like a bottom bracket that needs some good facing. Most cheap frames are not faced and will have paint on the outside of the bracket. This prevents the bearings from lining up neatly.

Properly faced bottom bracket (note how metal is showing for the full 360 degrees):

Nope, the frame was faced. The FSA spindle is slightly different than Shimano, making it a tight fit. Cheap frames or expensive frames, most don't get faced anymore. This was an expensive frame that was faced, at my urging, by a shop.

Last summer I helped my friend install an Ultegra BB with a FSA crank. The fit was "very" tight. We got it in with the use of a mallet (at my friend's urging, I cringed while doing it) and it has worked fine but the fit is less than ideal. I think it is hit or miss really, sometimes they fit fine, sometimes they need help.

I had the same experience when installing a Dura Ace BB with a FSA Gossamer crank. I had to use the mallet to push the crank spindle through the right bottom bracket dust cover, but it did not take more than a couple of hits.

Last summer I helped my friend install an Ultegra BB with a FSA crank. The fit was "very" tight. We got it in with the use of a mallet (at my friend's urging, I cringed while doing it) and it has worked fine but the fit is less than ideal. I think it is hit or miss really, sometimes they fit fine, sometimes they need help.

I had the same experience when installing a Dura Ace BB with a FSA Gossamer crank. I had to use the mallet to push the crank spindle through the right bottom bracket dust cover, but it did not take more than a couple of hits.

There are TWO versions of the MegaExo cranksets. One will work with Shimano BB's , the other won't even though they all use the same 24mm axle.

The ones that will work are the lower range Gossamer and Omega cranksets that use the same pinch clamp design as Shimano.

The higher range K-force and SL-K designs use a tension screw to lock onto the spline on the spindle. This requires a metal sleeve that comes with the MegaExo BB's to support this 'compression' inwards behind the bearings. Else you won't get your lock screw tight enough or, you will get it so tight, the BB bearings will cease.

So, do differentiate between the TWO versions of the cranksets before you attempt to swap between Shimano BB's and MegaExo BB's.

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